How to change the legend box width when plotting in R - r

Question

I use the following script to generate a legend in R. But the legend box is too small... how do I increase the box width?
legend("topleft", lty = 1, legend = c("Sub_metering_1","Sub_metering_2","Sub_metering_3"),col = c("black","red","blue"))

Answer 1

You are probably resizing your graph after you plot it and the legend. If that is the case, and you want to keep the box, one option would be to plot the graph, resize it, and then generate the legend. Perhaps a better option would be to size the window to the desired width to start with:
# on Windows, you can use the `windows` function. elsewhere, try quartz or X11
windows(height = 7, width = 3.5)
plot(hp ~ mpg, data = mtcars)
leg <- legend("topleft", lty = 1,
legend = c("Sub_metering_1","Sub_metering_2","Sub_metering_3"),
col = c("black","red","blue"),
#plot = FALSE,
#bty = "n")
)
You can also define exactly where you want the box to fall by providing a pair of x and y coordinates to the legend function. Those values would represent the upper left and bottom right corners of the box. The legend function will actually generate the coordinates for the upper-left hand corner of the box along with the width and height. By default it returns them invisibly, but you can assign them to an object, and If you use the plot = FALSE, option to legend you can capture those coordinates and modify them as you wish without actually plotting the legend.
windows(height = 7, width = 3.5)
plot(hp ~ mpg, data = mtcars)
legend(x = c(9.46, 31), y = c(346.32, 298),
legend = c("Sub_metering_1","Sub_metering_2","Sub_metering_3"),
col = c("black","red","blue"),
lty = 1)
The legend function will actually generate the coordinates for the upper-left hand corner of the box (that's where I got 9.46 and 346.62) along with the width and height of the box. By default it returns them invisibly, but you can assign them to an object, and if you use the plot = FALSE, option to legend you can capture those coordinates and modify them as you wish without actually plotting the legend.
plot(hp ~ mpg, data = mtcars)
leg <- legend("topleft", lty = 1,
legend = c("Sub_metering_1","Sub_metering_2","Sub_metering_3"),
col = c("black","red","blue"),
plot = FALSE)
# adjust as desired
leftx <- leg$rect$left
rightx <- (leg$rect$left + leg$rect$w) * 1.2
topy <- leg$rect$top
bottomy <- (leg$rect$top - leg$rect$h) * 1
# use the new coordinates to define custom
legend(x = c(leftx, rightx), y = c(topy, bottomy), lty = 1,
legend = c("Sub_metering_1","Sub_metering_2","Sub_metering_3"),
col = c("black","red","blue"))

Answer 2

Part of the legend width is determined by the longest width of the labels you use, which is calculated via strwidth. Below an easy example how to halve or double the size by using legend(..., text.width = ...).
plot(1)
text = c("Sub_metering_1","Sub_metering_2","Sub_metering_3")
legend("topleft"
,lty = 1
,legend = text
,col = c("black","red","blue")
)
strwidth(text)
# [1] 0.1734099 0.1734099 0.1734099
# half the length
legend("bottomleft"
,lty = 1
,legend = text
,text.width = strwidth(text)[1]/2
,col = c("black","red","blue")
)
# double the length
legend("center"
,lty = 1
,legend = text
,text.width = strwidth(text)[1]*2
,col = c("black","red","blue")
)